- Which one is more robust/stable/reliable? - Which has more 'social' capabilities? - Which has more connectivity? - Which provides more features (discussion, commenting, threading...)? - Which is more successful?

Never underestimate the power of simplicity. I often hear people who still don't use Twitter state that the reason they don't is that they don't understand it. Yet Twitter does only one thing, and that one thing is incredibly simple. Hence, if someone decides they want to start using it, they don't have to spend time reading documentation on all the features, much less a book on how to install, upgrade, and otherwise maintain it. If you want an application or service to spread virally, it has to either be so simple that any user can, within moments of first seeing it, know without a doubt that this is something even they can't screw up, or be designed such that nobody wants to be the last person in their group of friends to start using it.